The present invention relates to a sensor element carrying a piezoelectric element and a ceramic substrate with an envelopment space formed in it preferably used for the sensor element.
A sensor element carrying a piezoelectric element or the like is used for measurement of the viscosity of a fluid, detection of solid particles in a fluid, and detection of vibrations.
For example, a sensor element is disclosed (official gazette of Japanese Patent Laid-Open No. 8-201265) which measures the viscosity of a fluid by using the characteristic of a piezoelectric film or vibrational portion (hereafter referred to as piezoelectric film or the like) that the amplitude of the piezoelectric film or the like decreases when the piezoelectric film or the like contacts a fluid to be measured (hereafter referred to as fluid) and the fluid has a large viscosity and increases when it has a small viscosity and thereby, applying a voltage to the piezoelectric film to detect a current corresponding to amplitude or the like, and measuring the changes of loss factors, electric resistances, and reactances of the piezoelectric film.
In the case of the sensor element, when the viscosity of a fluid such as a sulfuric-acid solution has a correlation with the specific gravity or component concentration of the fluid, it is possible to measure the specific gravity or component concentration of the fluid. Therefore, for example, it is possible to measure the change of specific gravities of sulfuric acid used as the electrolyte of a lead acid battery in accordance with the change of viscosities of the sulfuric acid and obtain the charge or discharge state of the battery.
FIG. 2 shows an example of the state in which the sensor element 1 is set in a lead acid battery. A porous separator 14 is inserted between a pair of electrodes 13a and 13b of the lead acid battery and the sensor element 1 is held so as to be embedded in the separator 14.
By storing the sensor element 1 in a battery case and injecting an electrolyte into a liquid case under the state, it is possible to measure the viscosity of the electrolyte because the electrolyte permeates the separator 14 and the gap between the electrodes 13a and 13b is filled with the electrolyte, and the electrolyte leaking out of the gap enters an envelopment space 6 through communication holes 15a and 15b formed on a flat substrate 2 and contacts led a vibrational portion 8.
In the case of the sensor element, however, if a fluid to be measured is a fluid having a large viscosity such as an aqueous solution or sulfuric acid or a fluid having a low wettability with the ceramics forming the flat substrate, or the envelopment space of the flat substrate is communicated with the outside of the substrate through a small communication hole, a difficult problem occurs that the air in the envelopment space of the flat substrate is not easily replaced with the fluid.
Moreover, there is a problem that even after a fluid to be measured is sufficiently supplied into the envelopment space, a sufficient amount of fluid is not present nearby the envelopment space or even if the fluid is sufficiently present, the fluid once supplied is replaced with bubbles again when many bubbles are present.